Intercellular communication is often mediated by receptors on the surface of one cell, which recognize and are activated by specific protein ligands released by other cells. Members of one class of cell-surface receptors, known as receptor tyrosine kinases (RTKs), are characterized by having a cytoplasmic domain containing intrinsic tyrosine kinase activity (Schlessinger and Ullrich 1992). This kinase activity is regulated by the binding of a cognate ligand to the extracellular portion of the receptor. The few RTKs known to be expressed in cell-type-specific fashions have been shown to play roles critical for the growth and differentiation of those cell types. For example, members of the neural-specific trk family of RTKs, which recognize the neurotrophin family of growth factors (Barbacid 1993; Glass and Yancopoulos 1993), are required for the survival and development of discrete neuronal subpopulations (Snider 1994).

We described a receptor-like tyrosine kinase that appears to be restricted to skeletal muscle in its expression, and named it MuSK for muscle-specific kinase (Valenzuela et al. 1995). The mouse MuSK ortholog has been termed Nsk2 (Ganju et al. 1995), and a potential MuSK ortholog has been cloned from the Torpedo ray (Jennings et al. 1993).